By design, internal combustion engines exhaust gaseous emissions. Such emissions generally include nitrous oxides (NOX), hydrocarbons (HC) and carbon monoxide (CO). Although the emissions are generally not harmful, increased environmental and health concerns on the part of state and federal legislators has resulted in the promulgation of regulations which place severe limitations on the maximum allowable exhaust emissions for varying classes of vehicles operative by internal combustion engines. It is thus desirable for vehicle manufacturers to produce vehicles which have exhaust emissions within prescribed limitations.
One known method of emission control involves the use of a catalytic converter to remove emission components before they are emitted into the atmosphere. Those skilled in the art will recognize, however, that efficient operation of such catalytic converters strictly depends upon proper adjustment of the air-to-fuel mixture used by the engine. For example, catalytic converters are known to have decreased efficiency for removing NOX from engine exhaust as air-to-fuel mixtures become lean. Similarly, converter efficiency is lost for removing HC and CO when engine air-to-fuel mixtures are rich. It is thus understood by those skilled in the art that maximum converter efficiency can be achieved in all respects when the air-to-fuel mixtures is at or near stoichiometric balance.
Devices for measuring the concentration of oxygen in a mixture of gases are disclosed in U.S. Pat. Nos. 4,897,174 to Wang, et al.; 4,915,814 to Harada, et al.; 5,039,972; 5,037,526; and 5,031,445 to Kato, et al.; and 4,990,235 to Chujo. Each of these patents describe a sensor whose electrical properties (e.g., voltage, resistance, etc.) vary in proportion to the concentration of the gas to be detected. This variation of electrical property can be measured and processed in order to generate a signal which can be used to control air-to-fuel ratio.
U.S. Pat. Nos. 4,891,122 to Danno, et al., and 4,891,121 to Hirako, et al., disclose a sensor unit comprising a detecting chamber with a sensing cell, a pumping cell and a heater in combination with interface circuitry for the generation of processed output signals. As disclosed in the Danno and Hirako references, the heater maintains the sensing cell and detecting chamber at the proper temperature. Closed-loop control circuitry uses the output of the sensing cell to control the pumping cell current. This control system pumps ionized oxygen alternately in and out of the detecting chamber to maintain stoichiometric balance. The magnitude of the pumping cell current is used to measure the deviation of the measured gas from stoichiometry and is measured through the use of undisclosed voltage detecting and adding circuits connected to a series current-sensing resistor. The sign of the current is used to indicate alternately an excess or lack of oxygen and is likewise measured via a voltage comparator.
Similar to the devices described by U.S. Pat. Nos. 4,891,122 to Danno, et al., and 4,89I,121 to Hirako, et al., universal gas exhaust oxygen sensors (UEGO) are devices which are used to sample the exhaust gas from an internal combustion engine. Those skilled in the art will recognize that such sensors also consist of a heater, a sensing cell and a pumping cell. In operation, the pumping cell current is controlled to regulate the sensing cell voltage to a constant value (typically 450 mV). The pumping cell current signal is then proportional to the air-to-fuel ratio of the combustion mixture. This signal is typically processed and directed to control circuitry which maintains the air-to-fuel ratio at the desired balance. The accuracy of the air-to-fuel ratio control, however, can be no better than the accuracy of the measurement of the pumping cell current.
Those skilled in the art will recognize that if the pumping cell current is measured by means of a series current sensing resistor, the value of the resistance must necessarily be small since any voltage drop across the resistance changes the voltage drop across the corresponding device being measured. Similarly, the current necessary to control the pumping cell must also be small. The voltage drop across the current sensing resistor must therefore also be a small quantity. In typical UEGO applications, a difference in the air-to-fuel ratio of 0.01 could correspond to a 150 microvolt change in voltage across the current sensing resistor. If the air-to-fuel ratio is controlled by means of a microprocessor or microcontroller interfaced through an analog-to-digital (A-to-D) converter, tremendous resolution would be required to react to such a change. For example, given a 0-to-5 volt input range, the number of discrete levels, L, required to be distinguished by the A-to-D converter would be: ##EQU1##
For the scenario above, this yields, ##EQU2## In the case of an N-bit A-to-D converter, N is related to the number of levels, L.sub.1, by the well known relationship: EQU 2.sup.n =L (3)
or, EQU n=log.sub.2 (L)=15.025 (4)
Rounding to the nearest integer, it is readily seen that a 16-bit A-to-D converter would thus be required to achieve this resolution.
A simpler solution exists. If the voltage signal which represents the pumping cell current is amplified prior to A-to-D conversion then less resolution is required since the Voltage Increment is proportionally increased by the gain of the amplifier. This amplification is a viable approach due to the wide variety of low cost operational amplifiers available. The only practical limitations are ensuring the peak-to-peak deviation of the amplified signal does not exceed the input range of the A-to-D converter and ensuring that the amplifier does not inject spurious signals of meaningful magnitude into the signal being processed. This amplification stage also provides an opportunity to perform shifting in the reference level which may be necessary due to the different voltage ranges of sensor signals and A-to-D convertor input signals.
One cause for concern and indeed a serious problem with respect to this amplification is the offset voltage induced on the output of an operational amplifier. Consider a Motorola MC1741C general purpose operational amplifier. The input offset voltage, V.sub.io, the effective voltage from the inverting to noninverting inputs of the amplifier when these inputs are grounded, is typically 1 millivolt. If the operational amplifier is connected in the inverting configuration, then the output voltage, V.sub.o, in response to a general input voltage, V.sub.i, is given by ##EQU3## where R.sub.f represents the feedback resistor from the output to the inverting input, R.sub.i represents the input resistor which couples the input voltage to the inverting input of the operational amplifier, the ratio of R.sub.f to R.sub.i is the voltage gain of the circuit, and the operational amplifier is assumed to be ideal in all respects except input offset voltage.
Considering the previous example where a 150 microvolt resolution is desired from an oxygen sensing signal, and assuming a voltage gain of 50 is chosen to reduce the number of bits required of the A-to-D convertor, then the output of the operational amplifier to a hypothetical 150 microvolt signal is equal to EQU V.sub.o =-7.5 millivolts (6)
in the ideal case where the input offset voltage is zero. If, however, the input offset voltage is equal to the typical value of 1 millivolt, the output voltage becomes EQU V.sub.o =-7.5 millivolts+51 millivolts=43.5 millivolts, (7)
a significant difference. In the application of air-to-fuel ratio control, this difference would cause the controller to believe that the mixture was rich when in fact, the mixture was lean. In overall effect, the air-to-fuel controller would attempt to regulate the output voltage to zero. This would require an input voltage of 1.02 millivolts. Thus, the controller would attempt to maintain the air-to-fuel ratio at a value approximately 1% from stoichiometry. This problem is exacerbated due to the presence of other offset errors within the operational amplifier including the input offset current, and the input bias current.
Theoretically, the effects of these induced offset errors could be eliminated by individually calibrating each amplifier using simple offset null circuitry. This solution would be costly in production and practically, the effects of these errors would remain since each of the offset quantities is dependent upon temperature. As the temperature of the interface circuitry drifts, the offset voltage would likewise drift from any preset calibration point. Over an automotive temperature range of -40.degree. C. to 85.degree. C. the drift of the offset voltages and currents due to temperature could be greater than the magnitude of the typical offset quantities at room temperature. The accuracy of the air-to-fuel ratio controller is limited by these effects.